The present invention is related to an UHF broadband antenna which is used in, for example, UHF band ground wave broadcasting and communications.
ISTB-T (Terrestrial Integrated Services Digital Broadcasting) programs have already been commenced since 2003 in limited regions of Japan, and are scheduled to be started in other regions of Japan until 2006.
As to the above-described Terrestrial Integrated Services Digital Broadcasting system, electromagnetic waves in UHF band are used, and a frequency range thereof is scheduled to be enlarged to 470 through 770 MHz.
Conventionally, two-element type dipole antennas are known as antennas which receive electromagnetic waves in the UHF band (refer to, for instance, Japanese Patent Publication No. 2003-273637A).
One conventional two-element type dipole antenna is constructed as shown in FIGS. 27A to 27C. In this type of antenna, first dipole elements 1a, 1b, and second dipole elements 2a, 2b, which are made of conductor pipes, are arranged in parallel to each other by spacing a predetermined interval. A center portion of these dipole elements is held by a retainer 3 made of an insulative material. The first dipole element 1a is electrically conducted to the second dipole element 2a by a metal plate 4a, and the first dipole element 1b is electrically conducted to the second dipole element 2b by another metal plate 4b. Then, electric power is supplied to feeding points 5 provided on the side of the first dipole elements 1a, 1b from a feeder 6.
FIG. 28 shows a horizontal polarization vertical plane directivity of the above-described two-element type dipole antenna at the frequency of 770 MHz. In the above-described two-element type dipole antenna, the higher the frequency thereof within the band is increased, the larger a difference between phases of electromagnetic waves becomes, namely one phase of an electromagnetic wave radiated from the first dipole elements 1a, 1b on the side of the feeding point, another phase of an electromagnetic wave radiated from the second dipole elements 2a, 2b on the side of the non-feeding point. As a result, in the case that the frequency range is broadened, as indicated in FIG. 28, a maximum value direction of the directivity (vertical plane) is tilted along a direction of an azimuth angle 90 degrees in the vicinity of an upper end frequency.
Generally speaking, the above-described two-element type dipole antenna is known as a broadband characteristic and a high gain characteristic. However, since this antenna is constituted by both the first dipole elements 1a, 1b and the second dipole elements 2a, 2b, which are made by employing the conductor pipes, there is such a drawback that a total number of structural components of this antenna becomes large, as compared with those of a half-wave length dipole antenna and of a biconical type antenna. Also, since a feeding impedance in the two-element type dipole antenna owns a broadband characteristic in the order of 200 to 300 Ω, an impedance conversion circuit is required in order to convert these high impedances into 75 Ω which is generally utilized.
Also, since the two-element type dipole antenna owns an unidirectional characteristic along such a direction defined from the first dipole elements 1a, 1b to the second dipole elements 2a, 2b, in such a case that both the first dipole elements 1a, 1b and the second dipole elements 2a, 2b are arranged within the same plane with respect to the electric field plane, this antenna element arrangement may cause a less electrical problem. However, in the case that the two-element type dipole antenna is used as a primary driven element, there are such problems that the two-element type dipole antenna becomes bulky, or the directivity and the gain characteristic of this two-element type dipole antenna are deteriorated, depending upon a mounting condition of the antenna.
FIGS. 29A and 29B show such an example that both the first dipole elements 1a, 1b, and the second dipole elements 2a, 2b are arranged within the same plane which is parallel to the electric field plane, in such a case that the above-described two-element type dipole antenna is used as a primary driven element. The two-element type dipole antenna is supported on a reflecting plate 7 via a supporting pillar 8.
In such a case that the first dipole elements 1a, 1b and the second dipole element 2a, 2b are arranged within the same plane with respect to the electric field plane, although the directivity is not deteriorated, there is such a problem that the two-element type dipole antenna becomes bulky, due to pressure of such distances between the first dipole elements 1a, 1b and the second dipole elements 2a, 2b. 
FIGS. 30A and 30B show such an example that both the first dipole elements 1a, 1b, and the second dipole elements 2a, 2b are arranged with the same plane which is perpendicular to the electric field plane, in such a case that the above-described two-element type dipole antenna is used as a primary driven element.
FIG. 31 shows a horizontal polarization vertical plane directivity of the two-element type dipole antenna shown in FIGS. 30A and 30B at a frequency of 770 MHz. Since both the first dipole elements 1a, 1b and the second dipole elements 2a, 2b are provided at the same distances from the reflecting plate 7, the resulting antenna can be made compact. However, since the two-element type dipole antenna owns such a directivity (vertical plane) as shown in FIG. 28, even when this two-element type dipole antenna is used as the primary driven element, a maximum value direction of the directivity (vertical plane) is tilted along such an azimuth angle direction of approximately 25 degrees in the vicinity of an upper end frequency. As a result, this two-element type dipole antenna owns such a problem that a gain thereof is excessively lowered, as compared with the dipole antenna shown in FIGS. 29A and 29B, namely, the first dipole elements 1a, 1b and the second dipole elements 2a, 2b are arranged within the same plane with respect to the electric field plane.
Also, since the above-described two-element type dipole antenna requires the length corresponding to approximately 0.5 λ (symbol “λ” indicates wavelength of used frequency) of the lower end frequency within the frequency band, if this dipole antenna is required to be made compact, then this 0.5λ-length requirement may constitute a problem.